REFRACTORIES 477 the so-called melting point of the refractory. One of the essential requirements of the test is that the rate of heating does not exceed 5°C. per minute, controlled preferably by the use of an optical pyrometer of the Morse or Holborn-Kurlbaum type.1 FKJ. 1.—Gas furnace for refractory testing. In gas-fired furnaces there is not much danger of heating the specimens too rapidly since the rise in temperature when approaching 3,000°F. is slow, but in electric furnaces this is a point which must be watched. i See p. -Mi).